Control apparatus having temperature responsive means and humidity responsive means responding to the temperature and moisture content of air for providing an output indicative of the enthalpy or "total" heat content of air have been commercially available for some time. One particular enthalpy control is sold by the applicant's assignee as the H205A enthalpy control and has a moisture responsive element connected between an adjustment member and a switch operator wherein the switch is operated as the moisture responsive element changes in length. A temperature responsive member is attached to the adjusting member to reset the position of the adjusting member in response to the temperature of a bulb. When such a control device is placed in the outdoor air inlet duct or an air conditioning system to be responsive to the temperature and moisture content of the air, the switch is operated in response to the enthalpy of the air. Such a control can be designed so the switch will operate over a predetermined characteristic curve approximating constant enthalpy. If the switch is connected to the damper control apparatus of an air conditioning system, when the enthalpy of the outdoor air is above some predetermined level, the outdoor air dampers can be closed to prevent the outdoor air from being drawn into the air conditioning system.
In such prior art apparatus wherein the temperature responsive apparatus resets the position of the adjusting member for the moisture responsive apparatus, at extremely high moisture levels, such as above 90 per cent relative humidity, the designs result in an operation of the switch at lower enthalpy than desired.
The present invention is concerned with a control apparatus providing an output indicative of a thermodynamic property of moist air of the enthalpy of air wherein the moisture responsive element which is reset by a temperature responsive element has a limit to prevent the control apparatus from being out of control at high moisture levels. Specifically, a mechanical connection is connected between the switch operator and the adjustment means in parallel with the moisture responsive element so that as the moisture responsive element increases in length beyond a predetermined value depending upon the high moisture level, the temperature responsive element is directly connected to the switch operator to control the switch in accordance with the temperature.